There has been a property measuring method of a laser ultrasonic type known as a method of measuring properties of metal, such as a crystal grain size.
This property measuring method of laser ultrasonic type irradiates an object of measurement by laser light to excite ultrasonic waves in the object of measurement, and measures variations in waveform of such ultrasonic waves, as they are propagating in the object of measurement, to thereby implement measurements of properties of the object of measurement.
There will be described the principle of the above-noted measuring method with reference to FIG. 13. From an exciting laser light source 82, pulse laser light is shot to an obverse side of an object of measurement M (typically, a metal), causing small-scale explosions (ablations) on the object of measurement M, thereby producing pulsed ultrasonic waves in the object M. Such ultrasonic waves propagate in the object of measurement M, attenuating, and appear as fine vibrations at a reverse side of the object of measurement M. To the reverse side, probing laser light is shot from a probing laser light source 84, and between reflected light of that and reference light, interferences are caused, using an interferometer 86, whereby fine vibrations appearing at the reverse side of the object of measurement M can be read as voltage signals. Those voltage signals are input to a computer 88, and waveform-analyzed, thereby obtaining crystal grain sizes, Young's modulus, Lankford value, etc.
Such being the case, by use of a property measurement apparatus of laser ultrasonic type, properties of an object of measurement M can be measured in a noncontact manner, enabling measurements even for objects of measurement with hot temperatures.
Further, the property measurement apparatus of laser ultrasonic type allows ultrasonic vibrations to be excited in an object M at high frequencies ranging several tens to several hundreds MHz, and is preferable in particular for measurements of crystal grain sizes of a rolled steel plate.
And now, such a laser ultrasonic measurement apparatus as described needs to employ laser light having sufficient intensities for irradiation of an object of measurement with exciting laser light to excite small-scale explosions (ablations) on a surface of the object of measurement, and for irradiation of the object of measurement with probing laser light to enable measurements by an interferometer. In particular, as a laser device for emission of exciting laser light, there should be a high-power laser device of Class 4 or more prescribed in the JIS (Japanese Industrial Standards C 6802 (1991) “Radiation Safety Standard for Laser Products”).
For use of such a high-power laser device, intensities of, among others, scattered light and reflected light of laser should be sufficiently reduced for a secured safety of workers. Therefore, for laser ultrasonic measurement apparatuses in the past, which were used in most cases in a sealed unmanned laboratory by remote operations, it practically was impossible to implement an application to a production line of a rolled steel plate, for example.
As a method of using a laser ultrasonic apparatus elsewhere than sealed laboratories, there has been a defect inspection method disclosed in Japanese Patent Application Laid-Open Publication No. 2004-101189. In this inspection method, light shielding covers are used to cover simply optical paths of laser light and irradiated parts, for a safety of workers to be secured.
However, in application of this method, where light shielding covers have to be attached close to an object of measurement, production lines for a rolling process of metal or the like might suffer from a difficulty to keep close attachment, for reasons such as that the object of measurement may have hot temperatures, that the object of measurement may move, and that in some cases the object of measurement may have uneven surfaces, with resultant leakage of scattered light of laser, reflected light, or such, as an issue.
Further, close attachment of light shielding covers to an object of measurement might cause among others surface flaws of the object of measurement, reduced temperatures of the object of measurement, and worn light shielding covers, as a disadvantage.
The present invention has been devised to solve such problems, and it is an object thereof to provide a laser ultrasonic property measurement apparatus without the need of close attachment of light shielding covers to an object of measurement, thus allowing for a facilitated transfer of an object of measurement to be put in or taken out.
Further, it is another object of the present invention to provide a laser ultrasonic property measurement apparatus allowing for a sufficient reduction in intensity of leaking scattered light or reflected light of laser, even for an object of measurement greater than a light blocking structure.